Efficient and Selective Photocatalytic Oxidation of Benzylic Alcohols

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Efficient and Selective Photocatalytic Oxidation of Benzylic Alcohols with Hybrid Organic-Inorganic Perovskite Materials Haowei Huang, Haifeng Yuan, Kris Janssen, Guillermo Solís-Fernández, Ying Wang, Collin Y. X. Tan, Dries Jonckheere, Elke Debroye, Jinlin Long, Jelle Hendrix, Johan Hofkens, Julian A. Steele, and Maarten B.J. Roeffaers ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00131 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

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ACS Energy Letters

Efficient and Selective Photocatalytic Oxidation of Benzylic Alcohols with Hybrid Organic-Inorganic Perovskite Materials Haowei Huang,† Haifeng Yuan,*‡ Kris P.F. Janssen,‡ Guillermo Solís-Fernández,‡ Ying Wang,§ Collin Y. X. Tan,† Dries Jonckheere,† Elke Debroye,‡ Jinlin Long,§ Jelle Hendrix,‡ Johan Hofkens,*‡ Julian A. Steele,† and Maarten B. J. Roeffaers*† † Department of Microbial and Molecular Systems, Centre for Surface Chemistry and Catalysis (COK), KU Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium ‡ Department of Chemistry, Faculty of Sciences, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium § State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou 350002, Fujian, P. R. China Supporting Information Placeholder

ABSTRACT: The impressive optoelectronic performance and low production cost of metal halide perovskites have inspired applications well beyond efficient solar cells. Herein, we widen the materials engineering options available for the efficient and selective photocatalytic oxidation of benzylic alcohols – an industrially significant reaction – using formamidinium lead bromide (FAPbBr3) and other perovskite based materials. The best performance was obtained using a FAPbBr3/TiO2 hybrid photocatalyst under simulated solar illumination. Detailed optical studies reveal the synergetic photophysical pathways arising in FAPbBr3/TiO2 composites. An experimentally supported model rationalizing the large conversion enhancement over the pure constituents shows that this strategy offers new prospects for metal halide perovskites in photocatalytic application. Table of Contents

Selective photocatalytic oxidation of benzyl alcohol to benzaldehyde using a FAPbBr3/TiO2 hybrid system.

Organic-inorganic halide perovskites (OIHPs) have emerged as an extremely exciting family of semiconductor materials, garnering strong promise for a range of photonic applications. This stems from several advantages of OIHPs over other well-established semiconductors, such as low-cost facile processing, tunable bandgaps, and superior charge transport properties.1 Early interest for OIHPs was sparked by their immediate success in efficient light harvesting.2,3 In just a few years perovskite-based solar cell devices have rocketed to remarkably high power conversion efficiencies, exceeding 22%.4 While novel photon-electrical and electrical-photon energy conversions using OIHPs continues to embody main stream perovskites research, their application in photocatalytical and photoelectrochemical processes remains relatively unexplored. Among many candidates for semiconductor photocatalysts, titanium dioxide (TiO2) is at present a prominent choice within both industry and fundamental research, due to its wide optical bandgap (380 nm), efficient photoactivity however limited to UV, high stability, and low cost.5 On the other hand, OIHPs offer strong potential to extend the efficient harvesting and utilization of light well into visible wavelengths. Yet the inherent instability of organic halide perovskites when exposed to moisture make them generally incompatible with typical photocatalytic reactions in aqueous media, like water splitting, CO2 reduction, and organic waste decomposition. Recently, Nam et al.6 overcame this limitation by using an oversaturated aqueous HI solution, and allinorganic CsPbBr3 and CsPbI3 quantum dots7,8 have been applied in the photocatalytic reduction of CO2

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and the oxidation of PEDOT respectively. Considering how the interesting properties of OIHPs (high solar absorption and good charge separation and transport) continue to offer inexpensive solutions to a wide variety of optics research problems, it is surprising that serious efforts to exploit OIHPs for improving important photocatalytic reactions is yet to be made. One of the extremely important industrial organic reactions is the selective oxidation of alcohols to carbonyls,9,10 which is conventionally performed by stoichiometric quantities of inorganic oxidant; notably dichromate and permanganate, noble metal catalysts Pt, Pd, Au, or organic oxidants such as 2, 2, 6, 6-tetramethylpiperidine oxide.10-17 Herein we build on the strong photoactivity of formamidinium (FA; HC(NH2)2+) lead bromide (FAPbBr3) perovskites for the highly efficient and selective photocatalytic oxidation of benzylic alcohols. By utilizing the relative band alignment of photo-generated charge carriers within in FAPbBr3/TiO2 hybrids,18,19 the best performance under simulated solar illuminated was obtained. The photophysical pathways of photo-excited carriers within the hybrid system are detailed and are found to benefit from truly synergetic effects; namely, the improved separation and transport of electrons and holes into their respective active regions. Our FAPbBr3, TiO2 and FAPbBr3/TiO2 samples are prepared using a simple room-temperature antisolvent precipitation method,20 with further procedural details provided in the Supporting Information. Pure FAPbBr3 and TiO2 crystallize into their thermodynamically stable cubic perovskite and anatase phases, respectively (powder X-ray diffraction (p-XRD), Figure S1).20-22 p-XRD of FAPbBr3/TiO2 consists of peaks from both phases, indicating successful synthesis of a FAPbBr3/TiO2 hybrid. Scanning electron microscopy (SEM) of the system further confirms this result, as shown in Figure S2. From the SEM micrographs, the average size of the pure FAPbBr3 is around 3 μm. In contrast, the average size of the hybrid material obtained via the same antisolvent precipitation however in presence of the anatase TiO2 as seed is about 180 nm, being one order of magnitude smaller than pure FAPbBr3. Besides their morphological differences, X-ray photoelectron spectroscopy (XPS) experiments (Figure S3) reveals no difference in the chemical state of the FAPbBr3 in the pristine or TiO2 mixed material. Rather, TiO2 influences the FAPbBr3 crystallization process yielding smaller crystals. The optical absorption edges of FAPbBr3, TiO2 and FAPbBr3/TiO2 are determined from UV-Vis diffuse reflection spectroscopy (DRS, Figure 1). Typical

bandgaps are seen here for both pure materials; 380 nm (3.2 eV) for anatase TiO2 and 580 nm (2.2 eV) for FAPbBr3. 21,23,24 Compared to bare TiO2, the absorption properties of 15% FAPbBr3/TiO2 is enhanced substantially in the visible, through the superposition of both characteristic absorption spectra. The absorption edge corresponding to FAPbBr3 in the mixed material is slightly blue shifted by 10 nm, relative to pure FAPbBr3, attributed to the reduced crystal sizes.24 FAPbBr3/TiO2 materials with relative compositions of 1, 5, 10, and 25 wt% FAPbBr3 are also synthesized, with their optical properties summarized in Figure S4. In accordance with increasing absorption volume, elevating the relative concentration of FAPbBr3 in the composite also sees the weight of the Vis absorption increase. Next, the photocatalytic properties of the full range of materials synthesised are evaluated under simulated solar illumination by applying the selective photocatalytic oxidation of benzylic alcohol in an apolar solvent (toluene) and using molecular oxygen. Their photocatalytic performances are displayed in Figure 2A, where the pure FAPbBr3 and TiO2 (10 mg) controls both exhibit a conversion rate of roughly 15 % (0.1mmol benzyl alcohol in 2.5 ml toluene), after 8 h illumination. However, their conversion selectivity to benzaldehyde differs from 99 % for pure FAPbBr3 to 95 % for pure TiO2 with benzoic acid as main side product. On the other hand, considering the performance of the FAPbBr3/TiO2 composite series in Figure 2A, a substantial enhancement is exhibited by the hybrid system, compared its pure constituents. The addition of only 1wt% of FAPbBr3 already leads to a doubling in activity compared to pure TiO2. The highest photocatalytic conversion of benzyl alcohol peaked at 63 % for the 15 wt% FAPbBr3/TiO2 composite, totalling a 4-fold enhancement over the pure controls. Further increases in the FAPbBr3 amount exceeding 15 wt% reduces the conversion rate. This can be attributed to two reasons: (1) the increased quantity of perovskite precursors leads to larger FAPbBr3 nanocrystals and aggregates resulting in longer distances to be travelled by the formed charge carriers and thus reducing the efficiency of charge separation; (2) the increased TiO2 surface coverage by perovskite materials also reduces the exposed surface area of TiO2 to the reagent solution. TiO2 plays an important role in charge separation and it reduces O2 to form the reactive superoxide radical, as described in scheme 1. As a result of both of these factors excessive amounts of perovskite will not necessarily lead to further improved photocatalytic performance.


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ACS Energy Letters

material is currently limiting the long-term stability of the catalysts and causes traces of Pb to leach into the solution. We remain optimistic that this problem can be resolved in the future by (1) minimizing the contact between the photocatalyst and the more polar reaction products, (2) further developments of lead-free OIHP photocatalysts.

Figure 1. UV–Vis DRS spectra of FAPbBr3, TiO2, and 15% FAPbBr3/TiO2 with the corresponding bandgaps shown in the inset.

Clearly, combining the two solid-state semiconductors dramatically increases its overall capacity for photocatalytic oxidation of benzyl alcohol, without compromising the excellent reaction selectivity (99 % across all composites). However, the potential influence of size effects is yet to be considered. To assess the possible role crystal size might play, nanocrystalline FAPbBr3 was synthesized using an altered synthesis procedure (see Supporting information for experimental and characterization details). Additionally, the synergetic role between both materials was evaluated by preparing FAPbBr3/SiO2 using the same antisolvent method and mechanical mixed FAPbBr3/TiO2-M. With these materials - controlling for the morphological parameters such as size and utilizing a mixed media - no discernible benefit was gained; full summary of the activity results can be found in Table S1. Thus, the enhanced photocatalytic performance exhibited by the FAPbBr3/TiO2 hybrid systems must ultimately manifest from an intrinsic synergy arising from the interplay between the composite and the interface.18,19 The stability of the materials when exposed to repeated reaction cycles is further tested. Figure 2B shows the results from both FAPbBr3 and 15% FAPbBr3/TiO2 following their reuse as a photocatalyst during five reaction cycles. While no reduction in activity is observed in the pure FAPbBr3 sample, the FAPbBr3/TiO2 composite slowly decreased in activity. Based on additional experiments (Figure S6) we conclude that the more polar reaction product, benzaldehyde, leads to a partial dissolution of the FAPbBr3, an effect that is accelerated by illumination. Therefore, the FAPbBr3 content in the hybrid composite reduces while more benzaldehyde is produced, resulting in a notable effect on the activity of FAPbBr3/TiO2. The slow dissolution of the perovskite

Figure 2. (A) Photocatalytic oxidation of benzyl alcohol over pure FAPbBr3, TiO2 and a series of FAPbBr3/TiO2 hybrids. (B) Recycle test for pure FAPbBr3 and 15% FAPbBr3/TiO2. Reaction conditions: benzyl alcohol (0.1 mmol), photocatalysts (0.01 g), solvent of toluene (2.5 mL) saturated with molecular oxygen, AM1.5G simulated light irradiation, irradiation time (8 h).

Interestingly, the ability for FAPbBr3/TiO2 composites to oxidise with high selectivity alcohols is not merely limited to benzyl alcohol. As summarized in Table S2 high selectivity is also realised when performing similar experiments on other aromatic alcohols. Additionally, we further tested other popular wide bandgap OIHPs, such as MAPbBr3 and CsPbBr3, in the selective photo-oxidation of benzyl alcohol.25 Comparable conversions are found on MAPbBr3/TiO2 and CsPbBr3/TiO2 composites, as summarized in Table S3. These results prove that the described photocatalytic strategy reported here is generic. Similar experiments on material involving iodine, MAPbI3 and MAPbI3/TiO2, however, were unsuccessful due to photo-oxidation of I- to form I3-, the Pb-I bond breakage and formation Pb-O bonds by oxygen intercalated in perovskite photocatalyst.20,21,26,27 Next, we shine light on the photophysical benefit of implementing a hybrid system – over its pure counterparts – by examining their important optical properties. Figure 3A shows the pronounced effect of TiO2 on the steady-state photoluminescence (PL) spectra of FAPbBr3. Pure FAPbBr3 exhibits the known weak PL emission centered around 560 nm, with a high energy shoulder near 545 nm we attribute the presence of smaller FAPbBr3 nanocrystals (see Figure



S2).28,29 On the other hand, 15% FAPbBr3/TiO2 shows a blue-shifted PL emission peaking at 545 nm, at the same position as nanocrystalline FAPbBr3 (see Figure S7), assigned to smaller crystallites with a larger bandgap (see Figure S8).24,28 This is consistent with the DRS and SEM results. More vividly, the PL intensity and PL quantum yield (PLQY) of FAPbBr3/TiO2 (PLQY=10.4%) is observed to increase significantly relative to pure FAPbBr3. In the presence of charge consuming photocatalytic reactions such as the oxidation of benzyl alcohol, the luminescence intensity strongly reduces (Figure S9).

drop in toluene after adding benzyl alcohol, indicating the free carriers to be involved in the photocatalytic reaction. During the benzyl alcohol photooxidation process, the photogenerated charge carriers are separated at the interface between TiO2 and FAPbBr3 and consumed in reactions, thus reducing the PL emission due to the electron-hole recombination. In addition, the extended free carrier lifetime is favorable for the interfacial transfer of carriers to reactive substrates to achieve a higher photo-activity.31 Under light irradiation, this composite forms a long lifetime excitation state to carry out the photoconversion from alcohol to aldehyde. In building a clear picture of how the photocatalytic behavior is so dramatically enhanced for the hybrid system, the different recombination pathways described above are summarized and identified in Scheme 1A and B.

Figure 3. (A) Steady-state PL spectra and (B) PL decay spectra of the prepared FAPbBr3 and 15% FAPbBr3/TiO2, including the corresponding multiexponential fitting parameters.

Time-resolved PL analysis is employed to unravel the charge carrier dynamics in the materials’ solid state, as shown in Figure 3B. The PL decays kinetics of FAPbBr3 and 15% FAPbBr3/TiO2 are agreeably fit with a three-exponential decay function, with their analyzed fit parameters contained in the inset; for completeness, a complete list of parameters used appears in Table S4. Three recombination lifetimes are assigned to distinct processes; trap-assisted (τ1), exciton (τ2) and free-carrier (τ3).24 Note that the difference in charge dynamics between the two decays in Figure 3B resides mainly in the exciton and freecarrier recombination, in that the contribution of the former is reduced from 23% in FAPbBr3 to 7% in FAPbBr3/TiO2 while speeding up from 42.7 ns to 23.9 ns. Conversely the free-carrier contribution increased from 75% in FAPbBr3 to 90% for the FAPbBr3/TiO2 hybrid system, while τ3 is extended from 266 ns to 459 ns, respectively. The suppressed exciton recombination and shorter lifetime indicates exciton dissociation at the FAPbBr3/TiO2 interface, a physical feature used to efficiently extract electrons in solar cell devices. 1,30 The extraction of electrons to TiO2 leaves holes in the FAPbBr3, increasing freecarrier PL emission, aligning with our previous observations. Moreover, the ratio of free-carrier emission (90%) in PL decay is in line with a PL intensity

Scheme 1. The experimentally supported mechanistic energy diagram of photo-induced charge transfer in (A) 26,32 FAPbBr3 and (B) FAPbBr3/TiO2. (C) Corresponding schematic of the proposed selective photocatalytic benzyl alcohol to benzaldehyde oxidation process over the FAPbBr3/TiO2 hybrid.

To further support this model, electron spin resonance (ESR) spectroscopy was performed to identify the generated reactive active oxygen species under solar light irradiation with 5,5-dimethyl-pyrroline Noxide (DMPO) is used as the trapping agent (Figure 4A). In dark, only negligible DMPO-˙O2− signals were recorded.33 When excited by solar light irradia-


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ACS Energy Letters

tion, however, the ESR signals of ˙O2− species in FAPbBr3 increase significantly, indicating that photogenerated electrons in FAPbBr3 reduce O2 to ˙O2−. Besides this ˙O2− species, the ESR spectra for the TiO2 and FAPbBr3/TiO2 hybrid show an additional signal coming from the nitroxide radical which is formed by the light-induced cleavage of the N-C bond and the ring opening of DMPO-˙O2− adduct.34 Because of the VB alignment, photo-generated electrons in FAPbBr3 can be efficiently funneled into TiO2, as evidenced by our PL results. Therefore, more nitroxide radicals are generated in FAPbBr3/TiO2, indicating more charge carriers are transferred to TiO2, leading to improved photocatalysis efficiency. In addition, the evacuation of photo-generated electrons from FAPbBr3 to TiO2 leaves densely populated photo-generated holes in FAPbBr3 that are available for oxidation reactions. We summarize the mechanism for the organic transformation over FAPbBr3/TiO2 in Scheme 1: upon solar light illumination, electrons and holes are generated in FAPbBr3 and electrons are injected into TiO2 to reduce the molecular oxygen absorbed on TiO2 into ˙O2− species, whereas the photo-generated holes remain in FAPbBr3 and oxidize the benzylic alcohol substrates to carbocations. The carbocations react with ˙O2− species to produce the final product, in corresponding to aldehydes.

Figure 4. DMPO-trapped (#) superoxide radical and (*) Nitroxide-like radical EPR spectra of (A) FAPbBr3, (B) TiO2 and (C) 15% FAPbBr3/TiO2 dispersed in toluene.

In summary, the band alignment in the FAPbBr3/TiO2 composites facilitates the efficient and highly selective photocatalytic oxidization of benzylic alcohols into aldehydes. By funneling the photo-generated electrons from the perovskite into TiO2, an excellent photoactivity and a more than 4-fold enhanced in photocatalytic efficiency realized. This work sheds light on applications of OIHP materials for broader photocatalytic reactions, not only reduction but also oxidation. We suggest an improved cyclability of such systems can involve an appropriate stabilizing capping agent and conclude that these

results ultimately widened the pallet for commercially viable photocatalytic semiconductor systems, using OIHPs/TiO2 hybrid composites. ASSOCIATED CONTENT Supporting Information. Experimental detail on materials synthesis and photocatalytic activity measurements, XPS, SEM, diffuse reflectance spectroscopy and photoluminescence are shown in SI. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author

[email protected] [email protected] [email protected] ORCID

Haifeng Yuan 0000-0001-6652-3670, Kris P.F. Janssen 0000-0002-2181-9635, Dries Jonckheere 0000-00021192-5004, Jelle Hendrix 0000-0001-5731-1297, Jinlin Long 0000-0002-3675-0941, Julian A. Steele 0000-00017982-4413, Johan Hofkens 0000-0002-9101-0567, Maarten B. J. Roeffaers 0000-0001-6582-6514. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge financial support from the Research Foundation-Flanders (FWO, grant # G.0962.13, G0B39.15, ZW15_09 GOH6316N, G.0B49.15, doctoral fellowship to G.S.F. and postdoctoral fellowships to H. Y., K.P.F.J., E.D. and J.A.S.), KU Leuven Research Fund (C14/15/053), the Flemish government through long term structural funding Methusalem (CASAS2, Meth/15/04), the Hercules foundation (HER/11/14) and the European Union (Horizon 2020) Marie Sklodowska-Curie innovation program (Grants 722591 and 641887 for PhD scholarship) to H.H and C.Y.X.T.. J.L. thanks financial support of NSFC (Grant # 21773031).

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Efficient and Selective Photocatalytic Oxidation of Benzylic Alcohols

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